Htop is a text-mode, interactive process viewer for Linux run via console/ssh, and to install htop in CentOS / Ubuntu is a pretty simple task.
Among other things,

This way you will notice what programs is using most RAM.

htop allows you to:

quickly view key performance statistics such as CPU(multi-core layout) and Memory/Swap usage;
scroll the process list vertically and horizontally to see all processes and complete command lines;
kill processes by selection rather than entering process number;
use mouse to select list items.

Installation

Centos / Redhat

yum install rpm

Ubuntu / Centos

sudo apt-get install htop

How to Use htop

in the terminal

# htop

also there are some other options for example the delay time that is -d